FIG. 13 is a diagram showing the configuration of a microwave oven as an example of the high-frequency heating apparatus. In the figure, the AC power from a commercial power supply 11 is rectified into a DC current by a rectifying circuit 13, then smoothed by a choke coil 14 and a smoothing capacitor 15 of the output side of the rectifying circuit 13 and applied to the input side of an inverter 16. The DC current is converted into a current of a desired high-frequency (20 to 40 kHz) by the on/off operation of the semiconductor switching elements within the inverter 16. The inverter 16 is controlled by an inverter control circuit 161 for driving and controlling the semiconductor switching elements which switch the current at a high speed, whereby a current flowing in the primary side of a boosting transformer 18 is switched in on/off states at a high speed.
The input current to the control inverter control circuit 161 is detected by detecting the primary current of the rectifying circuit 13 by a current transformer 17. The detected current is inputted into the inverter control circuit 161 and used for controlling the inverter 16. A temperature sensor (thermistor) 9′ is attached to a radiation fin for cooling the semiconductor switching elements. Temperature information detected by the temperature sensor is inputted into the inverter control circuit 161 and used for controlling the inverter 16.
In the boosting transformer 18, a primary winding 181 is applied with a high-frequency voltage outputted from the inverter 16 and a secondary winding 182 is applied with a high voltage in accordance with a winding ratio. A winding 183 having a small number of turns is provided at the secondary side of the boosting transformer 18 in order to heat the filament 121 of a magnetron 12. The secondary winding 182 of the boosting transformer 18 is provided with a voltage doubler rectifying circuit 19 for rectifying the output of the secondary winding. The voltage doubler rectifying circuit 19 is configured by a high-voltage capacitor 191 and two high-voltage diodes 192, 193.
When a microwave oven thus configured is operated in a state that a subject to be heated is not contained within a heating chamber at all or in a small heating load state, the temperature of the magnetron increases due to the back bombardment of the microwave and so ebm reduces. As a result, an anode current increases to thereby cause an overheating state due to a so-called empty heating or the small heating load, and so the temperature of the magnetron and the high-voltage diodes may increase largely than the normal state. If such a state is ignored, the magnetron and the high-voltage diodes may be broken by the heat.
As a method of preventing such a trouble, there is a method in which a thermistor for detecting the temperature is placed near the magnetron, the semiconductor switching elements, the high-voltage diodes etc. and the device is stopped to prevent the increase of the temperature before the thermal breakage of these parts thereby.
As a method of preventing the temperature increase, for example, Patent Document 1 discloses a method in which a thermistor is fastened to a radiation fin by means of a screw to thereby detect the temperature from the radiation fin (see Patent Document 1).
FIG. 14A shows the attachment method described in Patent Document 1 and also shows a state that the thermistor is fastened to the radiation fin by means of the screw. The radiation fin 7 for heat radiation is attached on a printed board 6, and the thermistor 9′ is attached just above a semiconductor switching element 8 attached near the radiation fin 7.
The heat radiation portion of the semiconductor switching element IGBT8 generating high heat is fixed to the radiation fin 7. The three legs of the element are inserted into the through holes of the printed board 6 and soldered on the opposite side of the board. The thermistor 9′ is also fastened to the radiation fin 7 by the screw and takes out the temperature information of the radiation fin 7.
Further, there is a method of attaching a radial thermistor near a semiconductor switching element of a printed board (see a patent document 2). FIG. 14B is a diagram showing the attachment method of Patent Document 2.
In this figure, a radiation fin 7 for heat radiation is attached on a printed board 6, and a semiconductor switching element 8 is attached in adjacent to the radiation fin 7. A thermistor 9′ is attached so as to oppose to the semiconductor switching element 8 via the fin.    Patent Document 1: JP-A-2-312182    Patent Document 2: Japanese Patent No. 2892454